A primary requirement for polybutylene terephthalate (PBT) resin used for molding, especially for blow molding, is sufficient melt strength to resist deforming under its own weight. Melt strength of a particular PBT resin is generally dependent upon the molecular weight of the resin and the amount of chain-branching present within the resin. Resins with lower molecular weights and no branching have lower melt strength than resins of higher molecular weight or resins with a significant degree of chain-branching in the resin structure.
Melt visocisity imposes an upper limit on the molecular weight of PBT that can be produced in a melt polycondensation reaction. Therefore, higher molecular weight grades of PBT are generally produced by solid state polymerization. This can be done by heating the solid PBT polymer in a finely divided form at a temperature just below its melting point in a vacuum or in an inert gas stream. It has, however, been found uneconomical to advance the molecular weight of PBT by solid state polymerization beyond an intrinsic viscosity of about 1.3 deciliters per gram (dl/g) measured in orthochlorophenol at 25.degree. C. Accordingly, other techniques for advancing PBT molecular weight have been described. German Pat. No. 2,400,097 for instance involves coupling PBT molecules through their hydroxyl or carboxylene groups with smaller molecules such as diepoxides or acid anhydrides. The effectiveness of the latter method depends on the concentration and accessability of reactive end groups.
While PBT of relatively high intrinsic viscosity and PBT with some chain-branching in its structure has been produced, there is a need for a process by which previously formed completely linear PBT can be modified to increase its molecular weight and form a significant amount of chain-branching within the structure of the resin.